Obese (ob/ob) mice maintained at 25-30 degrees require 40% less energy for maintenance than lean mice. This reduction in maintenance energy requirement is responsible for the improved efficiency of energy retention in obese mice. Obese mice accumulate less muscle than lean mice. The fractional breakdown rate of muscle in obese mice, estimated from 3-methylhistidine excretion, averaged 8.5% per day and in lean mice 5.0% per day. Skeletal muscle of obese mice contained fewer Na ion, K ion-ATPase enzyme units than muscle of lean mice. Thyroxine administration increased Na ion, K ion-ATPase in obese mice to a greater extent than in lean mice.